1. Field of the Invention
The present invention relates to an image forming apparatus which uses an electrophotography method or an electrostatic method, such as a copying machine or a printer.
2. Related Background Art
In recent years, image forming apparatuses, such as copying machines and printers, of a cleanerless type have been put into practical use. This type of apparatus eliminates or significantly reduces waste toner by recycling residual toner using a developing apparatus after large transfer.
FIG. 1 is a schematic view of an example of a cleanerless image forming apparatus.
A reference numeral 104 denotes a rotating drum type electrophotography photosensitive body (photosensitive drum), serving as an image bearing body. The photosensitive body 101 is turned counterclockwise as indicated by an arrow at a predetermined circumferential speed. A reference numeral 102 indicates a magnetic brush member (magnetic brush charger), serving as a contact charging member, which brush member is disposed, a charging location a being formed with a magnetic brush in contact with the photosensitive drum 101. A charging bias is applied to the magnetic brush member 102 by a bias power supply S1 to uniformly charge the rotating photosensitive drum 101 by contact so that predetermined polarity and a predetermined potential are provided.
The uniformly charged surface of the rotating photosensitive drum 101 is subjected to image exposure L by image exposing means, not shown, as electrostatic latent image forming means, thus reducing the charging potential of the exposed bright part of the photosensitive drum surface, so that an electrostatic latent image corresponding to an image exposing pattern is formed on the photosensitive drum surface.
A reference numeral 103 indicates a developing apparatus, which develops the electrostatic latent image on the surface of the rotating photosensitive drum 1 to form a toner image ta. A reference numeral 103a indicates a developing member, such as a developing sleeve roller or a developing roller, to which a predetermined bias is applied. The developing member 103a and the photosensitive drum 101 are opposite to each other to provide a developing location b.
A reference numeral 104 indicates a transferring roller (conductive roller) serving as transferring means, which is pressed against the photosensitive drum 101 in a predetermined way to form a transferring location (transferring nip) c. A transferring sheet P is fed to the transferring location c from a sheet feeding mechanism, not shown, at predetermined control times, clamped at the transferring location c, and conveyed. While the transferring sheet P passes the transferring location c, a predetermined transferring bias, which is opposite in polarity to charged toner, is applied to the transferring roller 104 by a bias power supply S3 to electrostatically transfer the toner image ta on the photosensitive drum 101 onto the transferring sheet P. A combination of reference characters, tb, indicates the toner image transferred onto the transferring sheet P.
After it passes the transferring location c, the transferring sheet P is separated from the photosensitive drum 101, conveyed to fixing means, not shown, subjected to toner image fixing therein, and ejected as a print.
As the photosensitive drum 101 rotates, residual toner tc deposited on the drum due to toner image transfer onto the transferring sheet P goes through the charging location a to the developing location b, where the residual toner is cleaned (recovered) from the drum by the developing apparatus 102 simultaneously with developing.
Cleaning residual toner from the drum simultaneously with development is to recover the residual toner tc left on the photosensitive drum 101 due to image transfer, using fog removal bias (DC voltage applied to the developing means and fog removal potential difference V back, or difference between photosensitive drum surface potentials) at the developing apparatus 103 during development in the following steps. The treatment is performed in parallel with image forming steps, such as charging, exposure, development, and transfer, if the image area is longer in the direction of rotation of the photosensitive drum 1 than the circumference of the photosensitive drum 1. This allows the residual toner tc to be recovered at the developing apparatus 103 and used in the following steps, thus eliminating waste toner. In addition, because no cleaning apparatus is necessary, space is significantly saved and the apparatus can markedly be reduced in size.
Contact charging apparatuses, that is, apparatuses which bring a contact charging member to which a voltage is applied into contact with a body to be charged to charge the body, have advantages over corona chargers of non-contact type because of a small amount of ozone produced, low electric power consumption, and so on. Thus charging apparatuses of such a type have been put into practice as charging means for a body to be charged, such as a photosensitive drum, in an image forming apparatus.
A magnetic brush member is preferably used as a contact charging member.
A magnetic brush member is electrically conductive magnetic particles which are magnetically bound on a sleeve, containing a magnet, to provide a magnetic brush. When it is stationary or rotates, the magnetic brush member is brought into contact with a charged body, and a voltage is applied to the member to start charging.
Alternatively, a fur brush member, a brush into which electrically conductive fibers are formed, or an electrically conductive roller (charging roller), a roll into which electrically conductive rubber is formed, is preferably used as a contact charging member.
Using an organic photosensitive body having a surface layer over which electrically conductive particles are spread as an image bearing body or an amorphous silicon photosensitive body in addition to a contact charging member as described above allows a charging potential almost equal to the DC component of a bias applied to the contact charging member to be provided on the surface of the image bearing body. Such a charging method is called xe2x80x9cinjection chargingxe2x80x9d. Using a small amount of electric power, injection charging can be performed without producing ozone because discharging is not done, using, for example, a corona charger to charge a body to be charged. This charging method has been a focus of attention.
However, when image forming is repeated, using a cleanerless image forming apparatus as described above, the preceding image is slightly left, that is, a xe2x80x9cpositive ghostxe2x80x9d occurs because residual toner is not recovered by developing means. A positive ghost is caused by the part of the photosensitive drum surface which is not charged under the residual toner tc on the photosensitive drum 101 when the toner passes the charging location a. This phenomenon is distinctive if the contact charging member 102 is contaminated.
In charging the part of the photosensitive drum surface under the residual toner tc using the contact charging member 102, it is essential to remove the residual toner tc from the photosensitive drum 101 during charging and return the toner to the photosensitive drum surface after charging to recover it using the developing apparatus 103.
To solve this problem, means for preventing a positive ghost is available which makes it easy to take the residual toner tc in the magnetic brush member 102 by applying a bias opposite in polarity to a charging bias by a bias power supply to an electrically conductive brush 105, auxiliary means for erasing the traces of the preceding image, which is disposed in contact with the photosensitive drum 101 upstream of the transferring location c in the direction of photosensitive drum rotation and downstream of the charging location a in the direction of photosensitive drum rotation.
The residual toner tc, which is conveyed from the transferring location c to a contact d between the surface of the photosensitive drum and the electrically conductive brush 105, often contains both charged toner with normal polarity (normal-polarity toner) and charged toner with reverse polarity (reverse-polarity toner). While it is on the surface of the photosensitive drum 101, reverse-polarity toner passes the contact d between the surface of the photosensitive drum and the electrically conductive brush 105 to the charging location a because the toner is equal in polarity to a bias applied to the electrically conductive brush 105.
On the other hand, normal-polarity toner and discharged toner are electrostatically taken in from the surface of the photosensitive drum 101 by the electrically conductive brush 105 to undergo primary collection. They are discharged and then recharged to reverse their polarity. The resulting toner with reverse polarity is ejected from the electrically conductive brush 105 against the surface of the photosensitive drum 101 and conveyed to the charging location a.
That is, the residual toner tc is given reverse polarity by the electrically conductive brush 105. The residual toner with reverse polarity is removed at the charging location a from the surface of the photosensitive drum 101 and taken in by the magnetic brush of the magnetic brush member 102 to mix the toner (secondary collection of the residual toner by the contact charging member). This causes the part of the surface of the photosensitive drum which is under the residual toner to be charged, thus preventing a positive ghost.
After undergoing secondary collection by the magnetic brush of the magnetic brush member 102, reverse-polarity toner is discharged and then recharged to return its polarity to normal. The resulting normal-polarity toner is electrostatically ejected from the magnetic brush against the surface of the photosensitive drum 101, conveyed to the developing location b, and cleaned simultaneously with development by the developing apparatus 103.
However, if even an image forming apparatus with the above-described electrically conductive brush 105, or auxiliary means for erasing the traces of the preceding image, repeats image forming or continuously forms images with a high image ratio, residual toner is deposited on the electrically conductive brush 105, thus increasing the resistance of the brush 105, so that the original object and advantage of the electrically conductive brush 105, that is, erasing the traces of the preceding image (preventing a positive ghost) by primary collection, discharging, reverse-polarity charging, ejection of normal-polarity toner and discharged toner in the residual toner are not provided.
It is an object of the present invention to provide an image forming apparatus which does not let image traces leave.
It is another object of the present invention to provide an image forming apparatus which prevents the resistance of auxiliary means.
It is still another object of the present invention to provide an image forming apparatus which can clean auxiliary means.
It is a further object of the present invention to provide an image forming apparatus which includes an image bearing body; charging means for charging the image bearing body; image forming means for forming an electrostatic image on the image bearing body, charged by the charging means; developing means for developing the electrostatic image on the image bearing body, using toner and recovering residual toner from the image bearing body; transferring means for transferring a toner image on the image bearing body onto a transferring sheet; auxiliary means which comes in contact with the image bearing body after transfer, so that a voltage opposite in polarity to a charge given by the charging means is applied to the auxiliary means; and cleaning means for cleaning the auxiliary means by producing such an electric field that toner goes from the auxiliary means to the image bearing body.
Other objects of the present invention will be clear from the following descriptions.